The Premium Plus actually is a synthectic. The Premium is a blend... Run the Premium Plus, in any brand, and any size. Also run Startron additive for ethanol gas. Even if they say it's non-ethanol, run it anyway. It's insurance against ethanol, and it's a stabilizer, so win-win.
The Premium Plus protects against ethanol and non-ethanol fuels, it doesn't clog jets and lean out your fuel, and it mixes better, also it's good for high performance motors both new and old. Can't think of any reason not to spend the extra money for it. It is an indesputible advantage for every single application. Are there cheaper alternatives for certain applications? Yes. But why chance it. I've run it in new and old. No load up at idle, minimal smoke thru the throttle range, and of course, no engine issues. All my most trusted local mechanics say the same thing... run Startron, and run Premium Plus Merc oil. Those who do have no issues, those who don't bring their motors in for rebuild. Sounds simple to me.